This invention relates to nuclear reactors and it has particular relationship to the repair of guide tubes of a nuclear reactor. The guide tube is part of the upper internals of a nuclear reactor. A guide tube is mounted above a fuel assembly of the core of the reactor and it serves to guide the control-rod cluster of an assembly and the control-rod drive as the cluster is displaced upwardly or downwardly with reference to the core. Each guide tube is made up of an upper section sometimes called "thermal sleeve" and a lower section called a lower guide tube and is positioned by, and held transversely to, the upper core plate of the reactor. Typically, a pair of bifurcated or split pins are suspended, spaced 180.degree., from the lower end of the guide tube. Each split pin is held by a nut which is locked to an upper counterbore in the flange of the lower guide tube plate. The split pin has a flange which engages the lower counterbore of the flange of the lower guide tube. The diameter of the flange of the split pin is slightly smaller (typically 0.0006 inch) than the diameter of the counterbore. The flange is forced into the counterbore by tightening up on the nut which engages the split pin. The tines or leaves of the pins resiliently engage walls of corresponding holes in the upper core plate of the reactor positioning the guide tube to receive the control-rod cluster as the control-rod cluster is moved upwardly or downwardly. The coolant of the reactor which bathes the pins is corrosive with respect to the pin material. It has been found that as a result of stress-corrosion cracking of a split pin, the hold-down nut for the pin may be released. Under the action of the coolant, the released nut may migrate through the upper internals into the piping and steam generator of the nuclear-reactor power plant and may damage the piping or the generator. In operation of a reactor, repair of a steam generator was required following discovery that a split-pin nut had migrated into the channel head of the steam generator.
The presence of cracks in split pins which may lead to failure can be determined by ultrasonic testing. Discovery of a crack in a split pin would indicate that corrective action be taken not only with respect to the guide tube involved but with respect to all guide tubes, typically about 60, in the reactor. The ultrasonic test is difficult to perform and has not proven itself entirely reliable. Corrective action is taken as to all guide tubes of a reactor if a nut is discovered in the channel head of a steam generator or in other parts of the reactor power plant. There are also reactors as to which it is anticipated that the split-pin nuts may fail. Corrective action is taken as to all guide tubes of such suspected reactors.
In accordance with the teachings of the prior art, it has been the practice, implemented in July and August of 1982, to replace the guide tubes as a whole. This practice has the disadvantage that the cost of replacement is high particularly in light of the fact that the guide tubes are composed of stainless steel and the pins of Inconel alloy and the cost of, and time consumed in, fabricating the guide tubes is high and new guide tubes are not readily available. A guide tube may cost as much as $80,000.00. A more important drawback is that the guide tubes are highly radioactive and disposal of so massive radioactive objects presents serious problems.
Another prior art practice which was developed and used in Japan in late 1978 and later, in early 1979, in France, is to replace only the split pins in a "hot" cell environment with non-robotic devices. This practice was further developed in France in late 1982 and early 1983. The old split pin is welded to the old nut. Typically the nut is composed of type AISI 304 stainless steel. The "hot" cell practice has the advantage that in a "hot" cell the new split pin can be welded to the nut. This practice has the drawback that it demands high personnel radiation exposure for long time intervals and is necessarily highly labor intensive and highly costly. The radiation levels may be as high as 1,000 REM per hour. An expedient which has been adopted is to decontaminate the whole guide tube but this is costly.
It is an object of this invention to overcome the disadvantages and drawbacks of the prior art and to provide apparatus and a method for correcting for the failure or potential failure of the split pins of the guide tubes of a nuclear reactor.